文档介绍：1 Prof. Yang, The college of Optic and Electronic Information Engineering, USST External Modulators Internal modulation Internal (or direct) modulation:The intensity of the radiated power is changed from maximum to minimum by simply changing the driving current . Two serious drawbacks: is restricted by the laser diode ’ s relaxation frequency. — the fast variations of the laser ’ s peak radiating frequency in response to a change in driving (modulation) current — results to produce a broadening of the light pulse. It is a serious limiting factor in high-munications 2 Prof. Yang, The college of Optic and Electronic Information Engineering, USST External modulation This approach leaves a laser diode to radiate a continuous light wave (CW) while a change in light power occurs outside the laser diode. We need to insert an ponent into the optical loop. Figure Block diagram of external modulation 3 Prof. Yang, The college of Optic and Electronic Information Engineering, USST Mach- Zehnder (MDM) external modulators [Lithium niobate (LiNbO 3 ) modulator]. Its principle of operation is as follows: The refractive index of lithium niobate changes when an electrical voltage is applied. Light splits equally when it enters the waveguide . When no voltage is applied, both halves of the incident wave have no phase shift and so they interfere constructively, forming the original wave. When voltage is applied, one- half of the incident wave experiences a phase shift of +90 ° because the refractive index of this portion of the wave-guide decreases, increasing the velocity of the light propagation and lessening the delay. The other half of the waveguide receives a -90 ° shift because its refractive index increases, reducing the velocity of the light propagation and lengthening the propagation delay. When the halves bine, they cancel one another. Such “ destructive ” interference demonstrates that we can